[PECOFF] Writer: Remove NULL padding at the end of each section.

If section size is not multiple of 512, the writer added NULL bytes at the end
of it to make it so. That is not required by the PE/COFF spec, and the MSVC's
linker does not do that too. So we don't need to do that, too.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197002 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Make a member functions non-virtual.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197006 91177308-0d34-0410-b5e6-96231b3b80d8
Use "static" instead of anonymous namespace.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197007 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Writer: Move SectionChunk's ctor inline.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197008 91177308-0d34-0410-b5e6-96231b3b80d8